Incidence is defined as the number of what, per population at risk, during a defined period?

• A. New cases in a defined period per population at risk
• B. Total cases at a point in time
• C. Proportion of population with exposure
• D. Mortality rate

Answer: (A)

Incidence measures the chance of developing a disease over a specific time period. It uses the number of new cases that occur during that period as the numerator and the population at risk (those who could become cases) as the denominator. This captures how many people who were disease-free become cases, which is the essence of incidence.

This differs from prevalence, which describes the total number of existing cases at a point in time or over a period, not just new ones. It also differs from measures of exposure prevalence, which relate to how many people have a certain exposure rather than disease onset. Mortality rate focuses on deaths, not new disease cases, so it’s a different outcome measure altogether.